Climatic and anthropogenic drivers of tree-line shifts in the Lesser Caucasus over the past 25 years revealed by tree-rings and remote sensing

Tree‑line shifts are a key response of forest ecosystems to global warming, especially in high‑mountain ranges where climatic gradients are steep. While rising temperatures would suggest upward migration, actual tree‑line dynamics are also modulated by water availability and historic land‑use intensity. We examined tree‑line changes in the Lesser Caucasus over the past 25 years using a combination of dendrochronological data from 15 high‑elevation sites and remote‑sensing images spanning the region’s diverse climates from humid subtropical conditions near the Black Sea to semi‑arid regimes in the southeast. The land-cover classification with Landsat 5, 8 and 9 imagery (30x30m spatial resolution) from the years 1998 and 2023 shows a general upward trend of tree-lines but with strong spatial variations: the humid northwestern part experienced advances of up to 2.2m/year, whereas the more arid southeastern sector recorded retreats of up to 1.2m/year. Tree‑ring width chronologies reveal a weak, positive relationship with winter and summer temperatures, indicating improved growth under a warming climate. Water limitation in tree-ring width is slightly stronger in the drier northeast of the Lesser Caucasus than in the more humid northwest. But the signal is generally weak, there is no clear hydroclimatic trend and the spatial differences may only reflect the uneven distribution of species across the sampling network. Interpretation of these findings suggests warmer summers under rather constant moisture regimes have permitted tree growth beyond current tree-lines. However, at mid‑ and low‑latitudes, tree-lines on south‑facing slopes are usually situated lower than on north‑facing slopes because water limitation - not thermal limitation - dominates on the sun‑exposed aspect. In our study area, we also observe lower tree-lines on south‑facing slopes, yet those same slopes exhibit the strongest upward shifts over the last three decades. Hence, tree-line dynamics cannot be explained by temperature or drought alone. The most plausible additional drivers include snow dynamics and the recent reduction of anthropogenic pressure (e.g., reduced grazing and illegal logging) that has enabled upslope forest expansion, especially on south‑facing slopes. Further monitoring of tree growth dynamics across the Caucasus region, and particularly in the southern Lesser Caucasus, where tree-ring data are currently lacking, would be essential to resolve the observed tree-line shifts and anticipate potential future changes.